Solar generation and cogeneration systems are becoming a logical alternative or addition to fossil fueled energy systems as fuel costs increase. The solar heat that is simultaneously collected with electricity provides a major boost to an energy system's value. Unfortunately, however “solar cogeneration” systems need to be located at the site of use, which presents challenges to most existing or previous photovoltaic concentrator methods. Since the collected heat generally is at low temperature (typically 40-80 degrees C.), the heat energy cannot be transmitted far without substantial parasitic losses. Further, the capital cost of hot water and other heat transmission systems favors direct on site use. And, such low temperature heat generally cannot be converted in a heat engine to mechanical or electrical power because of the small temperature differential versus ambient temperatures. Accordingly, systems are needed that harvest light energy and transfer the harvested energy easily to the heating, lighting and electricity requirements at the site of use, such that the immediate needs of the site are factored into how the system is controlled.
Solar cogeneration technologies are held back by challenges in building optical systems that are both inexpensive and that can be mounted or integrated into a building. One problem is the practical limit for how tall a design can be to withstand forces from windy conditions on the device and building on which it may be mounted. Tying a cogeneration apparatus into the foundation or load bearing structure of a building creates expensive installations and/or mounting systems to accommodate system stresses, particularly on the roof. Many commercial sites lack sufficient ground space for a reasonably sized system and roof-mounting is the only viable option to get sufficient collector area. Thus, systems are needed that can be built into or added on to existing buildings easily and that use inexpensive materials.
Several inventions address these needs but leave many problems unsolved. For example, U.S. Pat. No. 4,690,355 teaches the use of silvered mirror slats that are coordinately controlled, but such assembly is placed on a mast and left exposed to the elements. A high cost system designed to protect solar cells from solar wind while dissipating heat only as radiant energy in space is described in U.S. Pat. No. 5,180,441. This space-based system uses thick slats and very small solar cells, while relying on a much larger surface area to dissipate heat in the absence of air (no convective cooling). The system uses a very small solar cell area with high precision reflectors that do not easily accommodate changes in radiation angle. Multiple plate designs also are presented in U.S. Pat. Nos. 4,034,736 and 4,159,707, but these require multiple reflections of light to absorb energy. U.S. Pat. No. 4,143,640 likewise teaches a Venetian blind structure, but uses thick slats with heat transfer fluid inside, which likewise is impractical.
Other problems of solar harvesting systems arise from limitations of optics used, which can be classified broadly as either reflecting or refracting optics. Refraction optics is non-linear, which limits sunlight concentration when the incidence angle of incoming light varies with respect to an optical surface. Refracting optics require focusing to track sun direction in two axes, or in one axis combined with tilt of the entire system for the second axis, with comparatively expensive structural and mechanical implications. The use of such complicated tracking systems to orient optics at the sun in 2 axes (azimuth and elevation), generally require larger apertures (collector areas) to average the cost of the complicated tracking system into a device that collects more energy. This exacerbates challenge of roof mounting such 2-axis designs. Accordingly, any technology that alleviates the need to track sunlight in a second axis would be a great improvement and can advance this industry.